This invention relates to an improved process for polymerizing formaldehyde to a high molecular weight polyoxymethylene, and in particular to the use of an initiator which results in a product having very desirable physical properties.
High molecular weight polyoxymethylene is a well known commercial product. It can be made, for example, according to the process described in U.S. Pat. No. 2,994,687 (Goodman et al.), which involves passing dry, gaseous formaldehyde into a reactor containing a hydrocarbon solvent in which is dissolved a quaternary ammonium compound serving as the polymerization initiator. The polymer is isolated and its end groups are capped with acetic anhydride to stabilize the polymer against hydrolysis.
Various quaternary ammonium compounds can be used as initiators in this process, but salts of weak acids, especially of carboxylic aliphatic acids, are preferred because such salts are unlikely to cause polymer degradation or hydrolysis. The above-mentioned Goodman et al. patent lists a number of quaternary ammonium compounds which can be used in that process.
A typical polymerization initiator is dihydrogenated tallow dimethylammonium acetate (DHTA) which has the following formula (1) ##STR1## wherein each R is a hydrogenated tallow (primarily, 18 carbon atoms-containing) group.
Polymerization of formaldehyde often is carried out in the presence of a small amount of a molecular weight control agent, which can be, for example, water, methanol, or acetic anhydride. Each of these additives has certain advantages and disadvantages. For example, water gives the largest particle size but the lowest yield of the polymer. Acetic anhydride is potentially the most desirable because it caps the free hydroxyl groups right in the polymerization process and thus leads to the highest yields. On the other hand, acetic anhydride, when used in a hydrocarbon solvent, gives the smallest particle size. Yet, very small particles are difficult to handle, have a tendency to clog filters, reduce plant capacity, and require more complex drying facilities than larger particles.
It would, therefore, be of considerable industrial interest to be able to carry out formaldehyde polymerization in the usual solvents in the presence of acetic anhydride as the molecular weight control agent and still make a polyoxymethylene of acceptable particle size.